Preselectors, which can be constructed as reversers, coarse selectors or multiple coarse selectors, generally serve the purpose in an on-load tap changer for a control transformer with main winding and regulating winding of widening the regulating range of the control transformer, also termed tapped transformer. Reversers make it possible to connect the regulating windings with the main winding selectably in the same sense or in opposite sense to the main winding. Coarse selectors make it possible to connect a larger part of the main winding, i.e. a so-called coarse step winding or coarse step, selectably with the regulating winding or not, thus to bridge over the coarse step winding.
Not only the step winding of a control transformer, but also the coarse step are briefly electrically separated from the main winding during switching-over thereof by reversers or coarse selectors. In that case they are subject to an electrical potential that results from the voltages of the adjacent windings as well as the coupling capacitances with respect to those windings and with respect to grounded components. The resulting difference voltages are in part considerable; they load the switching paths of the opening preselector contacts and if at a relevant level can lead to impermissible discharges. Discharges of that kind, also called arcs, cause formation of gas in the insulating oil of the on-load tap changer. This is undesirable, particularly since with increasing system voltages the proportion of undesired gases increases.
DE 10 2009 025 358 describes a tap changer with a preselector for uninterrupted switching over between different winding taps of a regulating winding of a tapped transformer. The preselector comprises two fixed preselector contacts and can be constructed as a reverser or coarse selector. One fixed preselector contact is electrically connected with the start of the regulating winding and the other fixed preselector contact is electrically connected with the end of the regulating winding. A movable contact is provided, whose root terminal is electrically connected with the main winding of the tapped transformer and that selectably connects one of the fixed preselector contacts. The movable contact is constructed to be longitudinally displaceable and has a switching chamber hermetically sealed off relative to the environment and in which similarly longitudinally displaceable contact members are disposed. The contact members, when switched onto one of the fixed preselector contacts, are closed against the force of a spring, but are opened during switching over between the fixed preselector contacts. A disadvantage of this known tap changer is that the arrangement, drive and mounting of the switching chamber have a substantial constructional cost and require a large amount of installation space.